Precision ion separation via self-assembled channels

Selective nanofiltration membranes with accurate molecular sieving offer a solution to recover rare metals and other valuable elements from brines. However, the development of membranes with precise sub-nanometer pores is challenging. Here, we report a scalable approach for membrane fabrication in which functionalized macrocycles are seamlessly oriented via supramolecular interactions during the interfacial polycondensation on a polyacrylonitrile support layer. The rational incorporation of macrocycles enables the formation of nanofilms with self-assembled channels holding precise molecular sieving capabilities and a threshold of 6.6 ångström, which corresponds to the macrocycle cavity size. The resulting membranes provide a 100-fold increase in selectivity for Li+/Mg2+ separation, outperforming commercially available and state-of-the-art nanocomposite membranes for lithium recovery. Their performance is further assessed in high-recovery tests under realistic nanofiltration conditions using simulated brines or concentrated seawater with various Li+ levels and demonstrates their remarkable potential in ion separation and Li+ recovery applications.


Characterization techniques
Nuclear magnetic resonance (NMR). 1 H NMR and 13 C NMR spectra were obtained on a Bruker-500MHz spectrometer, operating at frequencies of 500 MHz for 1 H and 126 MHz for 13 C, respectively.The synthesized compounds were dissolved in D2O with the addition of deuterated hydrochloric acid (DCl).The chemical shift of D2O corresponding to the residual H2O signal is referenced at 4.70 ppm in the data analysis.

Cryogenic transmission electron microscopy (Cryo-TEM).
For Cryo-TEM analysis, 3 µL of LiOH-Am7CD solution (prepared as described in the subsequent LiOH-AmCD-TMC membrane preparation section) was vitrified with the use of Vitrobot Mark IV (Thermo Fischer Scientific, MA, USA) on a glow-discharged TEM grid (multiple hole diameter and spacing, 20 nm carbon C-Flat) (Protochips, NC, USA).Samples were analyzed with a Titan Krios G-2 CryoTEM microscope at 300 kV of accelerating voltage.

Fourier-transform infrared spectra (FTIR).
FTIR spectra were recorded using a Thermo Scientific Nicolet iS20 spectrometer to investigate the chemical composition of the LiOH-Am7CD-TMC membranes.To obtain an adequate sample quantity, free-standing nanofilms were consecutively extracted from the interface between a 1.23%wt Am7CD/0.03MLiOH solution and a 0.2 %w/v TMC Isopar G solution at 3-minute intervals.The collected material was meticulously washed with hexane and water, followed by thorough drying in a freeze-dryer for one day.ATR-FTIR spectra were acquired within the range of 500 to 3600 cm -1 , involving 32 scans, and water background signals were subtracted from the obtained spectra.

Atomic force microscopy (AFM).
AFM images were acquired by using the Dimension ICON scanning probe microscope.An RFESPA-75 etched silicon probe was used to gently tap the membrane surface in the ambient air with a scan rate of 1Hz.The AFM images were processed using Gwyddion 2.44 SPM software.The free-standing membranes, prepared as described in a subsequent section, were initially floated in water and then carefully torn to create a crack before being transferred onto silicon wafers.Afterward, all samples were thoroughly air-dried at room temperature.

Transmission electron microscopy (TEM).
For TEM analysis of self-standing thin films, the formed LiOH-Am7CD-0.05TMC self-standing film was transported to water and collected from the water surface using glow discharged PELCO ® TEM 200 mesh copper grid with formvarcarbon film (Ted Pella, Inc., CA, USA).Such collected films were dried and stored in a dry cabinet prior to analysis.The TEM analysis was done with Titan ST HR TEM (FEI Company, OR, USA) at 300kV accelerating voltage.

Scanning electron microscopy (SEM). Field Emission SEM (SEM) (Zeiss Merlin Electron
Microscope) was used to characterize the surface morphology and thickness of prepared thin film composite membranes and PAN support.The measurements were performed at an accelerating voltage of 5.0 kV and a working distance of 4 mm.Prior to SEM analysis, the samples were dried at room temperature under vacuum conditions and subsequently sputter-coated with a uniform Iridium layer (3 nm) using a Quorum Q300RT sputter coater.

Surface zeta potential.
To assess the surface charge characteristics of the prepared membranes, zeta potential measurements were conducted using a SurPASS TM 3 electrokinetic analyzer (Anton Paar, Austria) equipped with an adjustable gap cell designed for planar samples (20 mm × 10 mm).
Two distinct KCl electrolyte solutions were employed, i.e., 1 mM and 10 mM.For the 1 mM solution, tests were conducted across a pH range approximately spanning 2.5-10, while with the 10 mM concentration, the investigated pH range extended from 5.5 to 9. Throughout all measurements, the temperature was consistently maintained at 25 °C, and pH adjustments were automatically performed via the instrument's titration unit using 0.05 M NaOH and 0.05 M HCl solutions.

Inductively coupled plasma -optical emission spectrometry (ICP-OES).
The concentrations of ions in the permeate liquid were analyzed using ICP-OES (Agilent Technologies 5110).A calibration curve was established using a series of corresponding standard solutions.To verify the accuracy and consistency of the calibration curve, a second solution with known ionic content was analyzed, with the relative error being kept below 3%.The sample solutions were appropriately diluted with a 1%wt HNO3 solution to ensure that the ionic concentrations fell within the range of 0.2 ppm to 200 ppm.

X-ray photoelectron spectroscopy (XPS).
After a two-day vacuum drying period, the free-standing films transferred onto silicon wafers were subjected to further elemental analysis.XPS was carried out utilizing an Axis-Ultra DLD spectrometer with Al Kα radiation (hν=1486.6eV) under a base pressure of 3 × 10 -9 mbar.The binding energy data were calibrated using the C1s signal of aromatic carbon at 284.5 eV as a reference.The C1s spectrum was methodically deconvoluted into several characteristic peaks for detailed analysis.

Powder X-ray diffraction (PXRD).
The 2D PXRD measurement of crystalline Am7CD powder (preparation details provided in the subsequent section titled 'Single crystal growth and preparation of bulk polycrystalline powder') was conducted using ground crystals.This analysis was performed on a Bruker D8 Discover system equipped with IuS microfocus Cu-radiation (λ = 1.54184Å) and an Eiger2R_500K detector.The 1D XRD pattern was generated by integrating the data obtained from the 2D frame X-ray crystallography.A suitable sized single crystal was probed on a Bruker D8 Venture diffractometer equipped with an IuS microsource and a Photon II detector at a temperature of 120 K. Molybdenum (Mo) was used as the X-ray source with Kα radiation (λ = 0.71073Å).The crystal structure was initially solved with the SHELXT structure solution program using Dual Space and subsequently refined with the SHELXL implemented in Olex2 program package.All nonhydrogen atomic positions were refined anisotropically.The H-atoms were treated as riding with Uiso(H) values set at 1.2Ueq(C) for tertiary CH and secondary CH2-groups and 1.5Ueq(C) for OH.Notably, the structure contains two voids housing heavily disordered H2O-solvent molecules.
Consequently, a solvent mask was applied during the final stages of the least-squares refinement.
Supplementary Table 1 and 2 provide detailed structural information for C42H77N7O28•[solvents].
The crystal structure was validated using the checkCIF and no A-or B-level alerts were generated.LiOH-Am6CD-0.1 TMC Am6CD 0.1 3
The synthesis procedure follows Supplementary Fig. 1 and was adapted from the existing literature 1-4 .
Step 1: Synthesis of heptakis(6-deoxy-6-chloro)-β-cyclodextrin (Cl-βCD) The synthesis of Cl-βCD began with the addition of 8.3g β-CD to 150 mL ultra-dry DMF under a nitrogen atmosphere.Methylsulfonyl chloride (23.3 mL) was then introduced dropwise into the cyclodextrin solution with continuous stirring.The reaction proceeded for 48 hours at 70 °C, followed by solvent rotary evaporation at 80 °C.The residual material was dissolved in methanol and stirred for 30 minutes.Subsequently, a 30% wt sodium methoxide solution in methanol was added dropwise until a pH of about 7 was reached.After an additional 4 hours of stirring, the resulting suspension was poured into ice water.The precipitate obtained after centrifugation was washed with methanol until the supernatant became colorless.The precipitate was finally dried in an oven at 50 °C, yielding 7.3 g of Cl-βCD.
Step 2: Synthesis of heptakis(6-deoxy-6-azido)-β-cyclodextrin (N3-βCD) 4.23 g of Cl-βCD was dissolved in 200 mL DMAc and 26.7 mL water, followed by the addition of 4.77 g NaN3 under an N2 balloon.Then the temperature increased to 110 °C for 2 days.The resulting light orange solution underwent further concentration using a rotary evaporator at 80 °C.
Upon cooling to room temperature, the concentrated solution was carefully poured into a substantial volume of water at 0 °C, yielding a precipitate that was subsequently filtered.Then, water was used to thoroughly wash out any unreacted NaN3, followed by a single rinse with acetonitrile.The resulting material was then dried at 50 °C under high vacuum conditions, yielding N3-βCD in the form of a white to yellow powder (4.2 g).
Step 3: Synthesis of heptakis(6-deoxy-6-amino)-β-cyclodextrin (Am7CD) 14.1 g PPh3 was added to a solution of 3.02 g N3-βCD in 56 mL anhydrous DMF, followed by stirring for 3.5 hours until the bubbling ceased.The solution became turbid slowly when 21 mL concentrated ammonia (30%wt) was added dropwise.The suspension was maintained at room temperature for over 20 hours and then concentrated via a rotary evaporator at 50 °C.The concentrate was poured into a substantial amount of ethanol to precipitate.The precipitate was separated by filtration and washed thoroughly with ethanol and acetone to remove any unreacted PPh3.The isolated solid was dried at 50 °C under high vacuum for 24 hours to obtain a lightyellow powder (1.65 g).Preparation of HAc-Am7CD-TMC thin film composite (TFC) membranes 144.0 mg synthesized Am7CD was added into 14mL of distillated water.16 µL acetic acid was then added to the suspension, resulting in a clear solution with a pH of approximately 7.4 during sonication (referred to as HAc-Am7CD solution, 9.1 mmol/L).A PAN support was immersed in this aqueous solution for 10 minutes, and any excess solution on the surface was removed using a rubber roller.Then, an organic solution containing TMC (0.3%w/v) was poured on the soaked support and allowed to react for 12 minutes.After washing with hexane, the membrane was dried in an oven at 80 °C for 10 minutes.The membrane was air-dried overnight at room temperature to remove any residual Isopar G.The resulting membranes were subsequently soaked in distilled water until testing.It is worth highlighting that all HAc-Am7CD-0.3TMC TFC membranes utilized in filtration measurements, surface z-potential and SEM characterization, as well as the free-standing membrane samples used for XPS, GIWAXS and AFM analyses, were manufactured with a 12-minute reaction time.The extended reaction time of 28 minutes was exclusively applied to the free-standing membrane samples intended for TEM measurements.This prolonged reaction time was implemented to increase membrane thickness and prevent damage caused by the electron stream during TEM analysis.

Preparation of free-standing membrane samples
The aqueous solution containing amino cyclodextrin (at the same concentration as in the previous sections on TFC membrane preparation) was poured into a glass vial, followed by gently introducing the organic solution containing TMC along the inner wall of the vial.After a specified reaction time (3 minutes for LiOH-Am7CD-TMC membranes; 12 minutes or 28 minutes for HAc-Am7CD-TMC membranes), the solutions were extracted using a syringe, and the resulting film at the liquid-liquid interface was deposited onto a pre-placed silicon wafer substrate in a vial.
Subsequently, hexane was employed for rinsing the membrane at least five times to remove residual acyl chloride.The membrane was then floated on water to eliminate unreacted amino cyclodextrin before being transferred to various substrates, including silicon wafers for Grazing-Incidence Wide-Angle X-ray Scattering, Atomic Force Microscopy, and X-ray Photoelectron Spectroscopy measurements, as well as PELCO ® TEM 200 mesh copper grids with Formvarcarbon film for Transmission Electron Microscopy analysis.

Single crystal growth and preparation of bulk Am7CD polycrystalline powder
Two identical solutions were prepared individually by dissolving 86.7 mg of commercial Am7CD•7HCl in 7 mL of 0.03 M LiOH solution, followed by 20 seconds of ultrasonication.Then, the resulting yellowish solution was filtered using polyamide filters with a pore size of 0.45 µm.
Subsequently, one of the solutions was sealed and left undisturbed on a flat surface at room temperature (20 °C) for 1-2 days to allow the formation of micrometer-scale crystals.The other solution was stored overnight, ultrasonicated for 1 hour, and left undisturbed for 8 hours until the clear solution became milky.The suspension was finally centrifuged, washed five times with water, rinsed with ethanol, and dried in an oven at 80 °C.

Filtration tests of neutral solutes
Various neutral organic compounds with different Stokes radii (RS) were used to study the pore size of the different types of cyclodextrin-based composite membranes.bar of applied pressure.After each filtration cycle at room temperature (20°C), the first 15 mL of permeates were discarded and the samples were collected for the analysis of total organic carbon content, whereas the rejection rate was calculated using Equation 2. Here, CP represents the total organic carbon content in the permeate, while CF denotes the total organic carbon content of the feed.Please note that unless otherwise clarified, the error bars in all figures and supplementary figures represent the standard deviation of a sample, derived from a minimum of three independent experiments.log10 (Rs) = −1.3363+ 0.395 × log10 (Mw) (1)    where Rs is in nm and molecular mass Mw is in g•mol -1 .

Ion sieving nanofiltration in a dead-end filtration system
Ion sieving nanofiltration tests were performed using Sterlitech stainless steel cells (model HP4750) with a membrane effective area (S) of 13.8 cm².The tests were conducted at a pressure The tested membrane was initially compacted at 6 bar for 1 to 2 hours until the steady-state was reached.Subsequently, the applied pressure was reduced to 5 bar to perform filtration tests using saline solutions.After discarding the initial 10 mL of permeate, the samples were collected.The salt rejection rate (R) was calculated using Equation 2, where CP and CF represent the ion concentrations in the permeate and feed solutions, respectively.In single-salt filtration tests, ion concentrations were determined by conductivity measurements using a conductivity meter (Eutech, mod.CON 2700).This involved establishing a relationship between ion concentration and electrical conductivity using known standards.As a result, we could determine ion concentrations in the sample solutions based on their measured conductivities.For filtration tests involving binary-salt feed compositions, ion concentrations in the liquid samples were directly measured using ICP-OES (Agilent Technologies, 5110).To assess the membrane selectivity, the separation factor (SF) was calculated using Equation 3, where CMg, F, CLi, F, are the concentration of Mg 2+ and Li + in the feed, respectively; while CMg, P, CLi, P represent the concentration of Mg 2+ and Li + in the permeate.A higher SF value typically indicates greater selectivity.Flux (L m -2 h -1 ) and permeance (L m -2 h -1 bar -1 ) were calculated using Equation 4and Equation 5, respectively.
Here, V (L) was monitored via a computer-interfaced balance and it represents the total volume of the permeating fluid during a specific time interval (∆t, h) at the applied pressure (∆P, bar).

Low-pressure cross-flow filtration of diluted brines
Pure water permeance, permeate flux, and solute rejection were evaluated in a Sterlitech crossflow laboratory-scale filtration system (WA, USA) comprising three stainless steel cells in series, as illustrated in Figure 4b of the main text.The housing cells consist of a rectangular channel with an active membrane area of 20.6 cm 2 .In total, this gives 61.8 cm 2 of membrane area being tested, which provides a more reliable statistical analysis.The cross-flow filtration system comprises a high-pressure feed flow pump (Hydracell, Wanner Engineering, Inc., Minneapolis, MN), a stainless steel feed vessel, three flat membrane housing cells (model CF016), temperature control and data acquisition systems.The cross-flow rate in the filtration system was continuously monitored using a floating disc rotameter.Adjustments to this rate, as well as to the operating pressure, were achieved through the coordinated use of a bypass valve and a back-pressure regulator (Swagelok, Solon, OH).Additionally, the permeate flow rate was automatically recorded at 60-second intervals using a computer-interfaced balance, ensuring precise and consistent data collection.The temperature was controlled via a recirculating chiller (DuraChill, Polyscience, USA) with a stainless-steel coil immersed in the feed tank.
Prior to each experiment, the membrane was immersed in water overnight.Following the loading of the membrane sample into the cross-flow housing cell, the sample was compacted for 4 h at an applied pressure of 20 bar.The applied pressure was then lowered to a value of 5 bar, 10 bar, or 18 bar, respectively.The pure water flux, Jw,0, was calculated by dividing the volumetric permeate rate obtained at steady-state by the membrane area based on Equation 4, and the pure water permeability coefficient of the membrane, "A", was calculated from the value of Jw,0 (Equation 5).
Solute rejection tests were performed with a constant cross-flow velocity of 0.50 m/s, and the feed stream consisted of a diluted salt lake brine (the composition is presented in Supplementary Table 4).The total volume of the feed solution for each rejection test was 5 L and the overall duration of each rejection experiment was roughly 8 h.Each collected feed and permeate sample was analyzed by means of ICP-OES to determine the single ion observed rejections using Equation 2 and separation factor using Equation 3.

High-pressure cross-flow filtration of concentrated brine
To assess the performance of Am7CD membranes in practical Li + /Mg 2+ separation, three synthetic feed solutions closely mimicking the composition of real lithium-containing brines were adopted.
These solutions included concentrated seawater (typical brine from RO seawater desalination activities), diluted Imperial geothermal brine (diluted by a factor of 7), and diluted Lungmu Co salt-lake brine (diluted by a factor of 4), which are characterized by various Li + levels and composition.Each synthetic feed solution used in the measurements was prepared by dissolving the proper amount of analytical grade reagents in deionized water (D.I. water, Milli-Q® IX 7005, Merck Millipore, Darmstadt, Germany), as detailed in Supplementary Table 5.All separation experiments were performed with a cross-flow lab-scale system described in detail in our previous publications 6,7 .It comprises a high-pressure pump (Hydra-cell pump, Wanner Engineering, Inc., Minneapolis, MN), feed reservoir, membrane housing cell, temperature control, and data acquisition system.The housing cell consists of a plate-and-frame unit with a 7.7 cm long, 2.7 cm wide, and 0.3 cm high rectangular channel, resulting in a 21 cm 2 total active area of the membrane sample.In this experimental setup, the retentate stream was continuously recirculated back to the feed reservoir.Meanwhile, the permeate stream was collected in a vessel placed on a computerinterfaced balance, allowing for the calculation of water flux across the membrane by measuring the change in permeate volume over time.The crossflow rate was closely monitored using a flowmeter (model 1900, ASA, Sesto San Giovanni, Italy) and adjusted, along with the operating pressure, through a bypass valve and a back-pressure regulator (Swagelok, Solon, OH).
Temperature levels were constantly monitored using a probe thermometer and maintained through a recirculating chiller (Model MC 1200, Lauda, Lauda-Königshofen) equipped with a stainlesssteel coil submerged in the feed tank.
Separation experiments were conducted with adjustments to the operating conditions tailored to each synthetic feed solution.Throughout all tests, the feed temperature was maintained at 25°C, and the cross-flow rate was set at 5 L/h.The applied pressure, however, varied depending on the specific synthetic solution, specifically 70 bar for concentrated seawater and 60 bar for Imperial geothermal brine and Lungmu Co salt-lake brine.At these operating pressures, each membrane underwent initial compaction using D.I. water until the permeate flux reached a stabilized state, typically overnight.Once the flux reached a steady state, the pure water flux (Jw) was determined using Equation 4.
Following the compaction stage, the synthetic feed solutions were introduced into the feed tank, and an initial feed sample was collected.The overall rejection performance of Am7CD membranes was evaluated by periodically collecting permeate and feed (recirculated retentate stream) samples as the recovery rates varied.Moreover, every collected feed and permeate sample underwent analysis via ICP-OES to determine the rejections of individual ions according to Equation 2, and separation factors were calculated using Equation 3. Notably, the endpoint of the separation experiments differed for concentrated seawater compared to the other two feed solutions.
Specifically, for the former, the test was terminated once oversaturation and salt precipitation commenced (typically occurring at recovery rates higher than 20-25%).Conversely, for the other brines, the endpoint was determined based on achieving either 50% recovery or fluxes dropping below 3 L m −2 h −1 , depending on which condition was met first.

Table 2 .
-1+x, +y, +z; 2 +x, -1+y, +z; 3 +x, +y, -1+z; 4 1+x, +y, 1+z; 5 1+x, 1+y, +z 137 Supplementary Crystal data and structure refinement for C42H77N7O28•[solvents]. 138 TMC in Isopar G with a specific concentration was applied to the support surface and allowed to react for 3 minutes.The membrane was then washed five times with hexane to remove residual TMC solution and subjected to a 10-minute thermal treatment at 80 °C.The membrane was airdried overnight at room temperature and stored in Mili-Q water at 4 °C before testing.The resulting membranes, varying in per-amino CD type and TMC concentrations, are detailed in Supplementary

Table 4 .
Composition of the synthetic feed solutions used in low-pressure crossflow filtration tests.

Table 5 .
Composition of the synthetic feed solutions, i.e., concentrated seawater, Imperial geothermal brine, and Lungmu Co salt-lake brine: ion concentrations and their ratios in relation to lithium.